Document the named subject feature.

Closes #1566
I think it's the reverse per [`memoized_helpers.rb#L338-L339`](https://github.com/rspec/rspec-core/blob/d97479a0/lib/rspec/core/memoized_helpers.rb#L338-L339):

```ruby
let(name, &block)
alias_method :subject, name
```

But yes, my example of `subject` using a block is incorrect. I just wasn't sure about exposing the `alias_method` call.